This is a timestamp which is automatically generated by syslogd, and is syslogd's way of stating that it has nothing to report. The default interval between two -- MARK -- lines is 20 minutes.

This can be changed by locating the syslodg startup script, and changing the parmeter after the "-m" option to suit your needs. To disable these timestamps completely, set "-m" to 0. For more information, type "man syslogd".